Why do network cards without all of the wires work?

Question

I recently acquired an older model of PC, and the network card in it did not work. A friend of mine had a spare laying around for me to use as a replacement, but I noticed that it only had 4 of the 8 pins to connect to the cable. He said this wasn't a problem and that it would work, but couldn't explain why (the non-functional one that came with the PC had all 8). I tested it out, and sure enough, it works. How can it work if it only has 4 of the 8 connectors?

UPDATE/EDIT:
Wikipedia states:

Conductors required
10BASE-T and 100BASE-TX Ethernet connections require two cable pairs. 
1000BASE-T Ethernet connections require four cable pairs.

Does it matter which two cable pairs are present? Or can any two pairs be used?

Answer 1

For 10 and 100 base T cable, only 2 pairs of wires are used. Some cable manufacturers save money by not including all the wires. 1000 base T requires all 4 pairs of wires.

Take a look at a standard land based telephone cord. Some have 2 wires, some have 4. Only 2 wires are needed to function, the other 2 are for a second phone line.

Check out the Wikipedia entry on Category 5 cabling for more detail.

To answer your modified question, yes, the pairs do matter. Look at this article. The transimt (tx) and receive (rx) wires are noted in the diagram.